Cooling towers are a widespread and effective heat transfer medium in industrial and utility type applications, thanks to the availability and high heat capacity of water. Cooling towers may be used to dissipate waste heat from air conditioning, industrial and power generation processes, among other uses. Proper maintenance of water quality within a cooling tower is important for the equipment to operate reliably, safely, and economically.
As water evaporates from the cooling tower and dissolved solids in the water begin to concentrate, there is a potential for microorganism growth. When microbial organisms reach a critical population, a slime layer or biofilm is formed. This slime layer contains both organic and inorganic matter. Some microorganisms excrete polymers and other protective substances which can encapsulate the microorganisms, protecting them from external environmental conditions. Biofilm growth generally occurs at interfaces between phases, for example, at an interface of air and water, and at an interface of water and a surface of the cooling tower.
As a result of biofilm formation, mixed populations of microorganisms can attach themselves to surface layers, causing the microorganisms to no longer be flushed away by cooling tower water flow and allowing for the growth of harmful bacteria, fungus, and macrobiota (e.g., protozoa including amoeba that support the growth of Legionella) that cannot otherwise survive in the bulk water. Biofilms protect microorganisms from disinfectants, causing water disinfection to be significantly more difficult when a biofilm is present. Within the protected biofilm layer, microorganisms can cause corrosion of the walls of the cooling towers and heat exchange systems. Moreover, biofilm creates an isolation layer on heat-exchange systems, causing them to no longer function properly and reduce efficiency in the heat exchanger, as well as blockage of filters, weirs, and screens in the system. The fill in a cooling tower can become so laden with biofilm that the entire cooling tower can collapse from the weight of the biofilm. Microorganisms present in the biofilm may accelerate oxygen uptake, causing an oxygen deficiency in the system. Some microorganisms may produce a number of organic acids, which causes a decrease in pH of the water in the cooling tower. Additionally, biofilms are a source of reinoculation (splash zones, dead spots, etc.) and reduced biocide performance. There are also potential health risks associated with biofilms in cooling water systems, such as Legionella, for example.
Reducing biofilm promotes the conservation of water, as more clean water is able to be recycled. Monitoring water quality improves the cycle number of water uses and reduces cooling system maintenance cost. Additionally, reducing biofilm on heat exchangers can also reduce the energy required to produce the same level of cooling.
Currently, it is common for service engineers to detect biofilm by sight or touch, at which point the biofilm is already well established. More scientifically robust methods are desired to detect biofilms before severe contamination occurs.
U.S. Patent Publication No. 2012/0085452 A1, published Apr. 12, 2012, discloses a corrosion coupon holder for fitting within a pipeline. However, monitoring and/or measurement of biofilm growth is different from monitoring and/or measurement of general corrosion not due to biofilm growth. Biofilm growth includes the adherence of live organisms to a coupon followed by extracellular polymeric substance (EPS) production and inorganic materials becoming entrapped in the biofilm EPS. Mature biofilm can be identified by the naked eye and scraped off of the coupon. Earlier stages of biofilm may be difficult to see by the naked eye, but organisms can be recovered from the surface of the coupon. Any corrosion resulting from the biofilm will be visible as deep pits form where galvanic cells were formed by the biofilm. While certain chemicals can create pitting corrosion, these pits will likely be more uniform in nature than the pitting corrosion from biofilm. The pitting corrosion from biofilm is generally not uniform in nature.
General corrosion is a uniform corrosion across the surface of the metal, and is measured differently from coupons. General corrosion is measured as the mil thickness of metal lost per year (mpy). When this measure is applied to any under deposit corrosion from biofilm, the mpy level is extremely low, even though there may be deep pits of corrosion. This is because metal is not being removed generally, but rather in specific locations. Thus, monitoring and measuring for biofilm growth and associated corrosion due to biofilm growth is different than monitoring and measuring for general corrosion that is not due to biofilm growth.